1. Field of the Invention
The present invention relates to a mobile communication system provided with a plurality of base stations, a plurality of mobile stations, and at least one radio network controller; and more particularly to a best cell amendment method by which the radio network controller determines the best cell for a mobile station and amends the best cell for that mobile station.
2. Description of the Related Art
The Third Generation Partnership Project (3GPP), a project committed to standardizing international standards for third-generation mobile communication systems, is investigating methods for using PDSCH (Physical Downlink Shared Channels) that are defined in, for example, the technical specification “3GPP TS 25.211 V4.0.0 (2001–03)” of the 3GPP, and methods for using HS-PDSCH (High-Speed Physical Downlink Shared Channels) that are defined in, for example, the 3GPP technical specification “3GPP TR 25.848 V4.0.0 (2001–03)” as methods for the high-speed transmission of data from a base station of a mobile communication system to a downlink line, which is a line to mobile stations within cells that are controlled by that base station.
PDSCH and HS-PDSCH are both used for transmitting data from a base station to a plurality of mobile stations. A plurality of users share one or a plurality of lines by means of the time division of this PDSCH and HS-PDSCH.
When using PDSCH to transport data, the base station sets Uplink Dedicated Physical Channels (UL DPCH) and Downlink Dedicated Physical Channels (DL DPCH) between each base station and the plurality of mobile stations for each cell in order to control data transfer to mobile stations that are in the cell that is controlled by the base station. UL DPCH is used by a mobile station to transmit user information or control information to a base station, and DL DPCH is similarly used by a base station to transmit user information or control information to a mobile station. In addition, each cell transmits a Common Pilot Channel (CPICH) as a pilot signal.
In this type of mobile communication system, a mobile station communicates by establishing a line with, of a plurality of base stations, the base station that has the best conditions. For this purpose, a radio network controller determines which of the base stations is the best base station (the best cell) for a plurality of mobile stations and reports the best cell to each mobile station. The mobile stations then each establish a PDSCH with the best cell that has been reported from the radio network controller.
However, a mobile station moves among a plurality of cells, and the best cell therefore changes over time along with this movement. The radio network controller must therefore determine the best cell for that mobile station at fixed time intervals, and when a best cell other than the cell that is currently set as the best cell becomes the best cell, a process is necessary for setting this new cell as the best cell.
Explanation next regards a method of the prior art for amending the best cell. The radio network controller sets a hysteresis margin as the control information and reports this hysteresis margin to each base station. The hysteresis margin is a threshold value for determining whether the best cell is to be amended. Each base station uses DL DPCH to report the hysteresis margin to mobile stations that are in cells managed by that base station. The mobile stations then measure the reception quality of the CPICH of the best cell and at least one other cell within a set time interval, and if the reception quality of at least one other cell is higher than the reception quality of the best cell, and moreover, if this difference is greater than the reported hysteresis margin, the mobile stations report information relating to the result of measuring reception quality to the radio network controller by way of the cell that has set the UP DPCH. The radio network controllers then amend the best cells of the mobile stations in response to these notifications from the mobile stations.
A data transmission method that uses HS-PDSCH has a larger volume of data transfer than a method that uses PDSCH and is referred to as High-Speed downlink Packet Access (HSDPA). Uplink and downlink control channels are also set for controlling the transmission of data from the base station to a mobile station when HS-PDSCH is used to transmit data. As with PDSCH, the amendment of the best cell is realized by comparing the reception quality of a downlink signal with a hysteresis margin.
In the method of the prior art for amending the best cells that was described in the foregoing explanation, there was the problem that the state of the load in cells having a high degree of congestion could not be improved when the hysteresis margin was large. There was also the problem that the inability to improve the degree of congestion resulted in a degradation of traffic balance. There was the additional problem that the amendment of the best cell after the occurrence of a large difference in the reception quality of a CPICH results in an increase in the amount of transmission power for transferring data, i.e., the transmission power per information bit, and a corresponding decrease in the data transfer capacity. Finally, when the hysteresis margin is small, cases do occur in which a mobile station moves from a cell in which the degree of congestion is low to a cell in which the degree of congestion is high, resulting in such problems as the degradation of traffic balance or an increase of the load upon the radio network controller due to frequent amendment of best cells.
In other words, the above-described mobile communication system of the prior art suffers from the problem that, because the hysteresis margin is a fixed value regardless of the degree of congestion, a state of poor traffic balance will remain unchanged without any improvement in the state of load in cells having a high level of congestion, even when cells having a high level of congestion are immediately adjacent to cells having a low level of congestion.
Japanese Patent Laid-Open No. H10-276463, which is directed toward solving these types of problems, discloses a mobile communication system in which an exchange that exercises control over base stations measures the amount of traffic in each of the base stations and amends the threshold value when initiating handoff processing in accordance with this amount of traffic, thereby impeding switching to cells that are controlled by base stations having a large amount of traffic and easing the switching from cells that are controlled by base stations having a large amount of traffic to other cells, and thereby controlling the distribution of traffic to achieve an optimum. In this case, traffic can be understood from the above-described patent document to mean the number of connected calls or the number of connected channels.
This type of mobile communication system determines congestion based on the volume of traffic and facilitates the switching of mobile stations of cells having a large amount of traffic to other cells. As an example, when a cell having a large amount of traffic is immediately adjacent to a cell having a small amount of traffic, setting the threshold value of the cell having a large amount of traffic to a lower value results in the switching of mobile stations in a direction that improves traffic balance. However, if the amount of traffic is high in both a particular cell and in an adjacent cell, the threshold values will remain high in both cells in this prior-art method of amending best cells, with the result that almost no switching occurs between the cells.
When cells having a large amount of traffic are adjacent, however, differences will occur in system throughput due to the distribution of users and the amount of data that is being transmitted by the base station. In such a case, system throughput is improved by using the above-described system throughput to determine the degree of congestion, and then switching mobile stations from cells having a high level of congestion to cells having even a slightly lower level of congestion. In the mobile communication system of the prior art disclosed in the above-described gazette, however, almost no switching of mobile stations is carried out, and an improvement in system throughput therefore cannot be expected. In this case, system throughput represents the number of bits that a base station that controls cells succeeds in transmitting per unit of time.
On the other hand, when cells having a small amount of traffic are immediately adjacent, the mobile communication system of the prior art disclosed in the above-described official gazette causes threshold values to be set uniformly low, whereby the frequency of the amendment of best cells increases even though almost no improvement in system throughput is obtained.